This invention relates to the field of electronic instruments for producing swept-frequency electrical signals and, more particularly, to such instruments that are capable of producing swept-frequency analog electrical signals over a predetermined range of frequencies. Specifically, one embodiment of the invention provides a method and apparatus for digitally correcting an analog ramp signal used to generate the swept-frequency analog signal produced by such an electronic instrument.
The design and implementation of radio-frequency (RF) devices, such as transmitters, receivers, filters, and test equipment, is facilitated by using an electronic instrument in the form of a test oscillator whose output frequency can be swept over a predetermined frequency range. Generally, there are two types of test oscillators, namely, analog sweepers and stepped sweepers. Analog sweep is preferable to stepped sweep in many instances, because there are no discontinuities in frequency as the output frequency of the test oscillator is swept. In contrast, in the case of stepped sweep, the output frequency of the test oscillator is incremented from one discrete frequency to another, and, consequently, a narrow frequency "glitch" in the response of an RF device under test may not evidence itself, if the glitch were to occur between discrete frequency steps.
Typically, in a test oscillator which produces a swept-frequency analog signal, the frequency is controlled by an analog voltage ramp. Therefore, any inaccuracy in the analog voltage ramp directly affects swept-frequency accuracy.
Known analog ramp generators that produce analog voltage ramps incorporate an analog integrator circuit to generate the analog voltage ramp to sweep the output frequency of the test oscillator. The sweep rate is typically controlled by a digital-to-analog converter (DAC) referred to as the sweep-rate DAC which is set by a central processing unit (CPU). A calibration procedure is required to correct the analog voltage ramp to achieve initial swept-frequency accuracy of the test oscillator. However, analog integrator circuits tend to drift with time and with variations in temperature during continued operation of the test oscillator.
One previous technique to correct for drift of the analog integrator circuit that causes swept-frequency inaccuracy during continued operation of a test oscillator is to include an analog-to-digital converter (ADC) to measure the end-of-sweep voltage, that is, the analog ramp voltage at the highest frequency of the predetermined range of frequencies over which the test oscillator is swept. The CPU reads the ADC, calculates the voltage deviation from a desired value, and adjusts the number loaded into the sweep-rate DAC on subsequent sweeps to provide correction.
There are three main disadvantages to this technique. One is complexity. ADCs are more complicated than DACs. Also, the CPU must be active at the end of every sweep to read the ADC and correct the setting of the sweep-rate DAC. Secondly, there is a one-sweep time lag between error measurement and correction. For example, the first sweep after operation of the test oscillator is initiated is uncorrected. Finally, although the start-of-sweep voltage is well-controlled and the end-of-sweep voltage is corrected using the ADC, the analog voltage ramp accuracy at intermediate points is uncorrected and depends on the linearity of the analog integrator circuit.
Therefore, a method and apparatus for providing correction of the analog voltage ramp are needed so that the swept-frequency accuracy of the test oscillator is improved. Moreover, such a correction desirably would be highly accurate and rapidly obtained.